The invention relates to fibers for plastic shrinkage crack control in cementitious compositions, and more particularly to a fiber composition that can be pumped and metered in a fashion similar to dispensing of fluid-type chemical admixtures, thereby facilitating verification of accurate dosage and achieving rapid and substantially uniform fiber dispersion within the mix.
It is known in the construction industry to add fibers into fresh cementitious mixtures to improve characteristics of the resulting hardened structure. Natural fibers (e.g. Sisal, cotton) and fibers made of steel, glass, carbon or synthetic materials are typically added into concrete to improve fracture toughness and to retard crack propagation. Concrete is prone to self-induced cracking, and, as it is a brittle material, these cracks propagate readily under relatively low stresses.
Self-induced, non-structural cracks occur in large masses of ready-mixed concrete or shotcrete due to small cracks that form early (e.g., cracks resulting from shrinkage of concrete or shotcrete during its plastic state), and these are subsequently propagated by stresses induced by changes in the dimension of such relatively large structures. Pavement concrete units, for example, are typically about 3 meters by 10 meters by 200 millimeters. Small cracks in such concrete can readily propagate, producing a weak link that results in subsequent fracture. This clearly visible cracking is often the only form of cracking that is perceived as being of importance, but it is a direct result of much smaller and probably essentially invisible earlier crack development.
In U.S. Pat. No. 5,399,195, Hansen et al. taught the use of very fine synthetic polymer fibers (e.g., polypropylene, polyethylene, polyester) for preventing self-induced cracks from developing due to plastic shrinkage of concrete at the micro-level. To promote substantially homogeneous dispersal of the fine fibers in the concrete using conventional mixing techniques and equipment, this patent disclosed that fiber bundles containing up to about 10,000 fibers (e.g., filaments, tapes) could be treated with conventional wetting agents to render them hydrophilic, thereby facilitating their introduction into concrete.
Other means are known for admixing fibers into concrete. For example, U.S. Pat. No. 4,961,790 of Smith et al. disclosed a soluble bag for introducing fibers into a concrete mix. In U.S. Pat. No. 5,224,774, Valle et al. disclosed non-water-soluble packaging that disintegrated due to the grinding action of concrete thereby achieving substantially uniform fiber dispersal. These enabled ready-mix truck operators to avoid having to handle loose fibers and facilitated their dispersal within the mix.
Nevertheless, there remains a need for verifying that plastic shrinkage control fibers are incorporated into the concrete mix at the correct dosage; and, secondly, to ensure that they are properly intermixed to avoid balling and to achieve substantially uniform dispersal throughout the concrete mix. Synthetic polymer fibers, particularly the fine fibers or fibrillated tapes used for plastic shrinkage control, are difficult to see in the wet mix. Not only might such fibers have transverse dimensions as small as 5-30 xcexcm, but they tend to be incorporated at low dosage rates due to their cumulatively large surface area. If clumping occurs beneath the concrete surface, these remain undetected; if clumps are found at the surface, these may be detected and removed for aesthetic reasons. In either case, the proper fiber dosage is not achieved; the risk of plastic shrinkage cracking increases.
Another major difficulty in verification is that fibers are added by the ready-mix truck operator who delivers the concrete to the construction site. The fibers are usually incorporated in dry batch amounts of 0.2-2.0 pound bags into the ready-mix trucks, wherein the bags disintegrate or dissolve to release the fibers into the concrete. Those ready-mix truck operators who are unscrupulous may use fewer than the prescribed amount of bags to pocket the difference and avoid the messiness, labor, and 10-15 minute delay necessitated per trip by this manual process. On the other hand, those ready-mix truck operators who are merely sloppy or negligent may fail to provide adequate mixing, may use the wrong fiber dosage or type, or may forget the fibers altogether. In each case, the risk of plastic shrinkage cracking increases (as does the ready-mix producer""s liability for faulty concrete).
Ideally, the present inventors believe that automated addition of fibers at a ready-mix plant would help to resolve the multi-faceted problem of verification and to minimize or eliminate expensive concrete ready-mix truck standing time. This is because fiber addition can be performed under certifiable conditions by the batch plant operator, who has customarily been responsible for ensuring that desired chemical admixtures (e.g., water reducers, set accelerators or retarders, pigments, etc.) are added into the concrete mix at prescribed dosage levels. In other words, the fiber addition could be automated and recorded by computer panel instead of being performed manually as is the present case. This would avoid having to rely on truck operators who might lack the requisite experience, training, honesty, concern, diligence, or patience (or all of the above) to perform the task properly.
At present, a number of bulk fiber system for automated dispensing of dry fibers are available. For example, Intech Corporation of Frederick, Colo., manufactures a system employing a vacuum to convey dry fibers into aggregate weigh hoppers, conveyor belts, or into concrete trucks. The system also purportedly has a capability for handling up to 300 pound bulk bags by automatic reloading. However, the present inventors believe that such bulk loading equipment may be too costly, complicated, and inconvenient for ready-mix plant operators to use or to integrate with existing chemical admixture dispensing equipment. This dry bulk fiber system does not provide verification that extremely fine, high-surface-area fibers of the kind used for plastic shrinkage control are sufficiently wetted out so as to achieve substantially uniformly dispersion in the concrete.
Accordingly, the present inventors believe that novel fibers and methods of fiber addition are needed.
In surmounting the disadvantages of the prior art, the present invention provides an aqueous fiber composition useful for dispensing fibers into matrix materials such as concrete or shotcrete, using pumping and metering equipment that permits automated control and verifiability of accurate dosage amounts. Although fibers made from synthetic polymers (e.g., polyolefin) are typically hydrophobic, suspending them in an aqueous fluid environment is believed to expedite their introduction into a wet concrete mix, to minimize clumping and facilitate intermixing, and to ensure substantially uniform distribution within the cementitious mix.
Thus, an exemplary fiber composition of the invention comprises a plurality of fibers, preferably made of synthetic polymers, glass, carbon or regeneratable sources, are suspended in an aqueous environment comprising water but essentially devoid of water-settable inorganic binder material, the fibers being present in the range of 5-40% based on weight of the. composition, the aqueous environment comprising at least one viscosity modifier for increasing the viscosity of the water within the aqueous environment, which has a Brookfield viscosity of 5,000-200,000 milliPascal*seconds measured at 25 degrees Celsius.
Exemplary methods of the invention comprise dispensing the above-described fiber composition into a cementitious mix, and preferably this is accomplished by pumping and metering the prescribed dosage of fibers into the mix. Accurate dosages of the fibers may thus be performed and verified by the concrete manufacturer.
Thus, the present invention provides the ability to pump and meter fibers at high speed into cementitious mixtures and to provide the ability to automate and to monitor the fiber addition. Morever, the fiber compositions of the invention are also believed to improve pumpability and cohesiveness of reinforced cementitious materials such as shotcrete and spray-applied fireproofing. In addition to use in ready-mix concretes, the fiber compositions are particularly useful in self-compacting concretes and mortars in which compaction and workability are issues.
Further advantages and features of the invention are further described in detail hereinafter.